In the German Pat. No. 540,863 a process has been described in which nuclei are prepared by adding an alkaline medium to a part of the titanium salt solution to be hydrolyzed; or by adding the titanium salt solution to a measured amount of the alkaline medium, and in either case in such a way that the mixture subsequently shows a pH value of 2 to 7, preferably 4 to 4.5. The mixture is subsequently maintained for some period of time at elevated temperatures ("cured"), the duration of this curing treatment and the temperature being dependent, to a great measure, on the pH value of the mixture.
A similar procedure has been described in U.S. Pat. No. 1,758,528. In this case the hydrogen ion concentration of a titanium salt solution, e.g., a titanium sulfate solution, is lowered by mixing with an alkali hydroxide or alkali carbonate solution to a pH value between 2.5 and 6, and thereafter curing the mixture.
Both procedures have disadvantages, namely, that even when the operations are carried out very carefully the nuclei so prepared are relatively inactive and must, for this reason, be added to the titanium sulfate solution in an amount of about 1% based on the titanium dioxide pigment to be made.
According to another U.S. Pat. No. 2,098,278 only enough alkaline-- reacting substance is added as is necessary to obtain a pH value between 1 and 2. In this procedure coarse nuclei are obtained; and up to 15% must be added to the titanium sulfate solution to be hydrolyzed.
Further, often, nuclei prepared according to the prior art can be employed only in the form of dilute suspensions with the result that the nuclei together with rather large amounts of liquid are introduced into the titanium sulfate solution to be hydrolyzed. The conditions in this solution are thereby drastically changed.